The present invention relates to an NB-WDM (Narrow Band Wavelength Division Multiplex) optical communication system, and particularly to an optical add-drop multiplexer (hereinafter abbreviated as OADM) to be employed in an in-line station of the NB-WDM optical communication system for adding or dropping optical signals of desired wavelengths among WDM optical signals transmitted in the system.
In order to transmit WDM optical signals for a long distance without quality degradation, it is important that optical intensity of every wavelength component of the WDM optical signal is maintained to be equal with each other at an appropriate level. In Japanese patent application No. 10-142046 filed by the applicant of present application on May 11, 1998 an entitled xe2x80x9cA WDM optical signal leveling circuit,xe2x80x9d there is a proposed an optical amplifier circuit to be used as a repeater node of a point-to-point NB-WDM optical communication system for amplifying and leveling optical intensity of every wavelength component at an appropriate level.
FIG. 3 illustrates an example of a point-to-point NB-WDM optical communication system, wherein optical amplifiers 16-1 to 16-m are inserted for repeatedly amplifying WDM optical signals to be transmitted from a transmitter terminal station 17 to a receiver terminal station 21.
The optical amplifier circuit as above mentioned is preferable to be employed as the optical amplifiers 16-1 to 16-m of such a point-to-point NB-WDM optical communication system. For organizing a flexible trunk network making use of the NB-WDM optical communication system, however, it is preferable that each node of the trunk network may be composed of an OADM which can drop or add optical signals of desired wavelengths from/to the WDM optical signals, for connecting the trunk network with local networks such as a SONET (Synchronous Optical NETwork), for example.
As to the OADM, there is a prior art apparatus disclosed in a Japanese patent application entitled xe2x80x9cAn Optical Add-Drop Multiplexerxe2x80x9d and laid open as a Provisional Publication No. 10-104461.
According to this prior art, WDM optical signals having n different wavelengths xcex1 to xcexn (n being a positive integer) are amplified all together by way of an EDFA (Erbium-Doped optical-Fiber Amplifier) and separated into n optical signals making use of n different optical-fiber gratings. Then, each (except one, for example) of the n optical signals is attenuated respectively through a variable attenuator to have an appropriate signal level, and multiplexed again into WDM optical signals by way of a 1xc3x97n star coupler. The one having a wavelength xcexi, for example, of the n optical signals is dropped and supplied to a SONET and another optical signal from the SONET is connected to the 1xc3x97n star coupler after its wavelength is converted into xcexi to be added into the WDM optical signals in place of the dropped signal.
However, in the prior art apparatus, the attenuation coefficient of each variable attenuator is determined initially according to the network design and no automatic level control is provided. This means lack of adaptability against change of operational condition. Furthermore, the number and the wavelengths of optical signals, or channels, to be dropped and added at each OADM are fixed in the prior art apparatus. This means lack of flexibility to variation of bandwidths to be assigned to local networks respectively, without saying of flexibility to the network configuration and its change of the trunk network.
Therefore, a primary object of the present invention is to resolve above problems and provide an OADM convenient for organizing an NB-WDM optical communication system, wherein the number and the wavelengths of the optical signals to be dropped and added can be controlled optionally and optical intensity of each wavelength component can be leveled automatically at an appropriate level.
In order to achieve the object, an OADM of the invention, of WDM signals having a plurality of wavelength components, comprises:
an optical demultiplexer for demultiplexing the WDM optical signals inputted to the optical demultiplexer into optical signals, each of the optical signals having respective one of the wavelength components and being outputted to respective one of optical paths;
an output-signal leveling unit for automatically controlling optical intensity of optical signals inputted through the optical paths, respectively;
at least one optical switches inserted in certain of the optical paths, respectively, each of the optical switches dropping a first optical signal inputted from the optical demultiplexer and connecting a second optical signal to the output-signal leveling unit when driven in an add/drop mode, and passing the first optical signal to the output-signal leveling unit when driven in a through mode; and
an optical multiplexer for multiplexing the optical signals outputted from the output-signal leveling unit into the WDM optical signals to be outputted.
Therefore, desired wavelength components of the WDM optical signals can be dropped and added by driving respective certain of the optical switches in the add/drop mode, and optical intensity of every wavelength component of the WDM optical signals can be leveled automatically at an appropriate level.